Invasive Cane Toads Research Articles

Invasive cane toads might initiate cascades of direct and indirect effects in a terrestrial ecosystem

Understanding the impacts that invasive vertebrates have on terrestrial ecosystems extends primarily to invaders’ impacts on species with which they interact directly through mechanisms such as predation, competition and habitat modification. In addition to direct effects, invaders can also initiate ecological cascades via indirect population level effects on species with which they do not directly interact. However, evidence that invasive vertebrates initiate ecological cascades in terrestrial ecosystems remains scarce. Here, we ask whether the invasion of the cane toad, a vertebrate invader that is toxic to many of Australia’s vertebrate predators, has induced ecological cascades in a semi-arid rangeland. We compared activity of a large predatory lizard, the sand-goanna, and abundances of smaller lizards preyed upon by goannas in areas of high toad activity near toads’ dry season refuges and areas of low toad activity distant from toads’ dry season refuges. Consistent with the hypothesis that toad invasion has led to declines of native predators susceptible to poisoning, goanna activity was lower in areas of high toad activity. Consistent with the hypothesis that toad-induced goanna decline lead to increases in abundance the prey of goannas, smaller lizards were more abundant in areas of high toad activity. Structural equation modelling showed a positive correlation between goanna activity and distance from dry season refuge habitats used by toads. The abundances of small lizards was correlated negatively with goanna activity and distance from dry season refuges of toads. Our findings provide support for the notion that invasions by terrestrial vertebrates can trigger ecological cascades.

Physiological plasticity in a successful invader: rapid acclimation to cold occurs only in cool-climate populations of cane toads (Rhinella marina).

Physiological plasticity may facilitate invasion of novel habitats; but is such plasticity present in all populations of the invader or is it elicited only by specific climatic challenges? In cold-climate areas of Australia, invasive cane toads (Rhinella marina) can rapidly acclimate to cool conditions. To investigate whether this physiological plasticity is found in all invasive cane toads or is only seen in cool climates, we measured the acclimation ability of toads from across Australia and the island of Hawai'i. We collected toads from the field and placed them at either 12 or 24°C for 12 h before measuring their righting response as a proxy for critical thermal minimum (CTmin). Toads from the coolest Australian region (New South Wales) demonstrated plasticity (as previously reported), with exposure to 12°C (vs. 24°C) decreasing CTmin by 2°C. In toads from other Australian populations, CTmins were unaffected by our thermal treatments. Hawai'ian toads from a cool, wet site also rapidly acclimated to cool conditions, whereas those from warmer and drier Hawai'ian sites did not. Thermal plasticity has diverged among populations of invasive cane toads, with rapid acclimation manifested only in two cool-climate populations from widely separated sites. Predictions about the potential range of invasive species thus must consider the possibility of geographic (intraspecific) heterogeneity in thermal plasticity; data from other parts of the species' range may fail to predict levels of plasticity elicited by thermal challenges.

Forecasting the spatiotemporal pattern of the cane toad invasion into north-western Australia

Context The toxic cane toad (Rhinella marina) has invaded over 50 countries and is a serious conservation issue in Australia. Because the cane toad has taken several decades to colonise northern Australia, due to the large size of the continent and the east–west invasion axis, there is scope for making testable predictions about how toads will invade new areas. The western toad invasion front is far from linear, providing clear evidence for heterogeneity in invasion speed. Aims Several ad hoc hypotheses have been offered to explain this heterogeneity, including the evolution of traits that could facilitate dispersal, and spatial heterogeneity in climate patterns. Here an alternative hypothesis is offered, and a prediction generated for the spatiotemporal pattern of invasion into the Kimberley Region – the next frontier for the invading toads in Australia. Methods Using observations of spatiotemporal patterns of cane toad colonisation in northern Australia over the last 15 years, a conceptual model is offered, based on the orientation of wet season river flows relative to the invasion axis, as well as toad rafting and floating behaviour during the wet season. Key results Our model predicts that toads will invade southern areas before northern areas; an alternative model based on rainfall amounts makes the opposite prediction. The models can now be tested by monitoring the spread of invasion front over the next 5–10 years. Conclusions Our conceptual models present a pleuralistic approach to understanding the spatiotemporal invasion dynamics of toads; such an approach and evaluation of the models could prove useful for managing other invasive species. Implications Although control of cane toads has largely proved ineffective, knowledge of the spatiotemporal pattern of the toad invasion in the Kimberley could: (1) facilitate potential management tools for slowing the spread of toads; (2) inform stakeholders in the local planning for the invasion; (3) provide researchers with a temporal context for quantifying toad impacts on animal communities; and (4) reveal the mechanism(s) causing the heterogeneity in invasion speed.

Methods for invasive species control are transferable across invaded areas.

Cane Toads (Rhinella marina) are invasive pests in many parts of the world, including the Japanese island of Ishigaki. Extensive research in Australia has identified promising new methods for control, but also has shown that toads exhibit geographic variation in many traits (suggesting that methods developed in one location may not work in another). Can the approaches developed in Australia play a useful role for controlling this invasive species in Japan? Our experimental trials on Ishigaki Island suggest that these new methods can be successfully applied to Japan. First, Cane Toad embryos exposed to chemical cues of conspecific tadpoles exhibited a reduction in viability (subsequent growth and development). This response appears to be species-specific, with native frog embryos not being affected by exposure to cues from toad tadpoles, and Cane Toad embryos not being affected by exposure to cues from native frog tadpoles. Second, Cane Toad tadpoles were attracted to traps containing water from conspecific eggs, and toxin from adult conspecifics. Third, adult Cane Toads were attracted to acoustic cues of calling males, with sex differences in rates of attraction to specific versions of a synthetic call (males were attracted to choruses whereas females were attracted to low-frequency calls). Our results suggest that the methods developed by Australian researchers are applicable to controlling invasive Cane Toads in Japan.

Out of the frying pan: Reintroduction of toad‐smart northern quolls to southern Kakadu National Park

AbstractInvasive species are a leading cause of native biodiversity loss. In Australia, the toxic, invasive cane toad Rhinella marina has caused massive and widespread declines of northern quolls Dasyurus hallucatus. Quolls are fatally poisoned if they mistakenly prey on adult toads. To prevent the extinction of this native dasyurid from the Top End, an insurance population was set up in 2003 on two toad‐free islands in Arnhem Land. In 2015, quolls were collected from one of these islands (Astell) for reintroduction. We used conditioned taste aversion to render 22 of these toad‐naïve quolls toad averse. Seven quolls received no taste aversion training. The source island was also predator‐free, so all quolls received very basic predator‐aversion training. In an attempt to re‐establish the mainland population, we reintroduced these 29 northern quolls into Kakadu National Park in northern Australia where cane toads have been established for 13 years. The difference in survival between toad‐averse and toad‐naive quolls was immediately apparent. Toad‐naive quolls were almost all killed by toads within 3 days. Toad‐averse quolls, on the other hand, not only survived longer but also were recorded mating. Our predator training, however, was far less effective. Dingo predation accounted for a significant proportion of toad‐smart quoll mortality. In Kakadu, dingoes have been responsible for high levels of quoll predation in the past and reintroduced animals are often vulnerable to predation‐mediated population extinction. Dingoes may also be more effective predators in fire degraded landscapes. Together, these factors could explain the extreme predation mortality that we witnessed. In addition, predator aversion may have been lost from the predator‐free island populations. These possibilities are not mutually exclusive but need to be investigated because they have clear bearing on the long‐term recovery of the endangered northern quoll.

Effects of invasion history on physiological responses to immune system activation in invasive Australian cane toads.

The cane toad (Rhinella marina) has undergone rapid evolution during its invasion of tropical Australia. Toads from invasion front populations (in Western Australia) have been reported to exhibit a stronger baseline phagocytic immune response than do conspecifics from range core populations (in Queensland). To explore this difference, we injected wild-caught toads from both areas with the experimental antigen lipopolysaccharide (LPS, to mimic bacterial infection) and measured whole-blood phagocytosis. Because the hypothalamic-pituitary-adrenal axis is stimulated by infection (and may influence immune responses), we measured glucocorticoid response through urinary corticosterone levels. Relative to injection of a control (phosphate-buffered saline), LPS injection increased both phagocytosis and the proportion of neutrophils in the blood. However, responses were similar in toads from both populations. This null result may reflect the ubiquity of bacterial risks across the toad’s invaded range; utilization of this immune pathway may not have altered during the process of invasion. LPS injection also induced a reduction in urinary corticosterone levels, perhaps as a result of chronic stress.

The costs of parasite infection: Effects of removing lungworms on performance, growth and survival of free‐ranging cane toads

Abstract Most research on the effects of parasites on their hosts has focused on the parasites of mammals or birds (especially, domesticated taxa) rather than systems in which the hosts are ectothermic wildlife species. We used experimental methods (antihelminthic drugs) to quantify the effects of lungworms (Rhabdias pseudosphaerocephala) on their anuran hosts, the invasive cane toad (Rhinella marina). In captivity, eradicating lungworms enhanced toad activity (measures of boldness and level of spontaneous activity), performance (locomotor speed, climbing ability) and foraging success (feeding rate). In free‐ranging toads (n = 123) at a site in tropical Australia, eradicating lungworm infection increased rates of host survival by 8%, movement by 20%, growth by 28% and elaboration of male secondary sexual characteristics by 30%. The presence of the lungworm thus has a substantial negative effect on fitness‐related traits of the host. Given their long shared evolutionary history and the mild inflammatory and immune response elicited by the parasite in the host, the magnitude of the effects of parasite removal were surprising. Parasites may impose hidden costs, related to modification of host behaviour or metabolism. Experimental removal of parasites can be a useful means of quantifying costs of infection. A plain language summary is available for this article.

Competition and pesticide exposure affect development of invasive (Rhinella marina) and native (Fejervarya vittigera) rice paddy amphibian larvae.

Increased pesticide use in rice agricultural ecosystems may alter competitive interactions between invasive and native amphibian species. We conducted an experiment with two rice paddy amphibians found in Luzon, Philippines, the invasive cane toad (Rhinella marina) and the endemic Luzon wart frog (Fejervarya vittigera), to determine whether exposure to a common herbicide, butachlor, drives competitive interactions in favor of the invasive amphibian. Our results revealed that competition had a strong effect on the development of both species, but in opposing directions; Luzon wart frog tadpoles were smaller and developed slower than when raised alone, whereas cane toad tadpoles were larger and developed faster. Contrary to our predictions, development and survival of endemic wart frog tadpoles was not affected by butachlor, whereas invasive cane toad tadpoles were affected across several endpoints including gene expression, body size, and survival. We also observed an interaction between pesticide exposure and competition for the cane toad, where survival declined but body size and expression of thyroid sensitive genes increased. Taken together, our findings indicate that the success of the cane toad larvae in rice fields may be best explained by increased rates of development and larger body sizes of tadpoles in response to competition with native Luzon wart frog tadpoles rather than lower sensitivity to a common pesticide. Our results for the cane toad also provide evidence that butachlor can disrupt thyroid hormone mediated development in amphibians, and further demonstrate that important species interactions such as competition can be affected by pesticide exposure in aquatic ecosystems.

Invasive cane toads are unique in shape but overlap in ecological niche compared to Australian native frogs.

SummaryInvasive species are an important issue worldwide but predicting invasiveness, and the underlying mechanisms that cause it, is difficult. There are several primary hypotheses to explain invasion success. Two main hypothesis based on niche spaces stand out as alternative, although not exclusive. The empty niche hypothesis states that invaders occupy a vacant niche space in the recipient community, and the niche competition hypothesis states that invaders overlap with native species in niche space. Studies on trait similarity/dissimilarity between the invader and native species can provide information on their niche overlap. Here, we use the highly invasive and well‐studied cane toad (Rhinella marina) to test these two hypotheses in Australia, and assess its degree of overlap with native species in several niche dimensions. We compare extensive morphological and environmental data of this successful invader to 235 species (97%) of native Australian frogs. Our study is the first to document the significant morphological differences between the invasive cane toad and a continent‐wide frog radiation: despite significant environmental overlap, cane toads were distinct in body size and shape from most Australian frog species, suggesting that in addition to their previously documented phenotypic plasticity and wide environmental and trophic niche breadth, their unique shape also may have contributed to their success as an invasive species in Australia. Thus, the invasive success of cane toads in Australia may be explained through them successfully colonizing an empty niche among Australian anurans. Our results support that the cane toad's distinct morphology may have played a unique role in the invasiveness of this species in Australia, which coupled with a broad environmental niche breadth, would have boosted their ability to expand their distribution across Australia. We also propose RLLR (Relative limb length ratio) as a potentially useful measure of identifying morphological niche uniqueness and a potential measure of invasiveness potential in anuran amphibians.

Evolutionary shifts in anti-predator responses of invasive cane toads (Rhinella marina)

A potential prey item’s response to encountering a predator depends on aspects of the predator (e.g., its locomotor capacity), the local environment (e.g., proximity to shelter) and the physiological state of the prey item, but in addition, anti-predator tactics also vary geographically among different populations within wide-ranging species. Using standardised trials, we tested responses of cane toads (Rhinella marina) to being placed on a laboratory runway and encouraged to flee. Overall, the toads least capable of rapid locomotion were the ones most likely to respond to simulated predator attack by exuding toxins rather than attempting to escape. A toad’s willingness to move down the runway and its propensity to exude toxin from the parotoid glands rather than fleeing were repeatable in successive trials and influenced by the animal’s location of origin, morphology, previous experience, and parentage. We found that specimens from Australia were more willing to flee than were those from the native range (French Guiana) or Hawai’i; larger toads, and those with relatively longer legs, were more willing to flee; captive-raised toads were less willing to flee and more willing to exude toxin. Captive-raised offspring resembled their wild-caught parents both in propensity to run and in propensity to exude toxin. Thus, geographic divergence among cane toad populations in anti-predator responses reflects a complex combination of processes, including both developmental plasticity and heritability. The process of colonising novel environments exposes an organism to a host of different predators that exert selection on morphology, behaviour and physiology. Over time, this can lead to the evolution of novel phenotypes that are adapted to local conditions. Anthropogenically facilitated biological invasions provide a unique opportunity to study how species respond to colonisation of new environments. Here, we studied geographic variation in cane toad (Rhinella marina) anti-predator behaviours from a range of locations (including invasive and native populations) using standardised trials. Our results suggest that invasive Australian cane toads have undergone a shift in their anti-predator response, becoming more willing to flee than are conspecifics from other areas. Location of origin, morphology, prior experience and parentage all played a role in shaping a toad’s anti-predator response, suggesting that the behaviour represents a combination of plastic and heritable variation.

Relative effectiveness of trapping and hand-capture for controlling invasive cane toads (Rhinella marina)

ABSTRACTManagement of invasive vertebrates is a crucial component of conservation. Management strategies should increase the chance of removal of every individual, by exploiting behavioural characteristics, and by increasing the period over which removal occurs. For example, traps can operate automatically over long periods, and often include attractants to increase captures. Management strategies for the invasive cane toad (Rhinella marina) in Australia include hand capture and trapping adult individuals (toads are attracted to an acoustic lure, and to insects attracted to a light, also on the lure). We used capture-mark-recapture analysis to compare the efficacy of trapping and hand capturing cane toads over 10 weeks, in Townsville, Australia. We trapped 7.1%–22.4% of the estimated population per week, and hand captured 1.7%–6% of the estimated population per week. Trapping was more efficient than hand capture in our regime; overall, more toads were caught per trapping man-hour than per hand-capture hour. Traps attract toads and maximise the period over which removal occurs; thus, the probability of removal for each toad was higher than by hand capture. Because hand capture and trapping seemed to remove different toads, a combination of these methods may work well.

High mortality and small population size prevent population recovery of a reintroduced mesopredator

AbstractNorthern Australia's native mammal fauna has undergone a severe decline in recent decades. Putative factors include altered fire regimes, cat predation, poisoning by cane toads and disease. Populations of northern quolls, Dasyurus hallucatus severely declined following cane toad invasion and have not recovered. We monitored a population of northern quolls in Kakadu National Park that was supplemented with ‘toad‐smart’ individuals, to determine whether cane toad poisoning or predation was preventing population recovery. The population increased after supplementation, but crashed in March 2012, coincident with a high level of trap disturbance by canids. Canid predation was the major source of mortality for radio‐tagged quolls. We used population viability analyses (PVA) to explore how changes in mortality influenced the likelihood of extinction. With no management, the quoll population has a 48% chance of extinction over the next 20 years. Sensitivity analyses highlighted small population size and high mortality as the main reasons for the population failing to recover. We then explored whether population supplementation or reducing mortality could increase the likelihood of persistence. One year of supplementation increased the probability of population survival over 20 years from 51.6% to 81.7%. Continuing supplementation for 3 or 5 years increased the probability of population survival to 96.5% and 98.1% respectively. Similarly, a 2.5% reduction in the rate of mortality for juveniles and adult females increased the probability of population persistence over 20 years to 83.6%. Further reductions in mortality of 5% and 10% increased the probability of survival to 92.2% and 99.4% respectively. The results of the PVA suggest that small interventions could have a significant positive effect on population survival. We hypothesize that predation from food‐subsidized canids is preventing the recovery of quoll populations. Future management actions to reduce mortality, via improved fire management, or through population supplementation, are necessary to ensure the persistence of the northern quoll.

Chronic effects of an invasive species on an animal community.

Invasive species can trigger trophic cascades in animal communities, but published cases involving their removal of top predators are extremely rare. An exception is the invasive cane toad (Rhinella marina) in Australia, which has caused severe population declines in monitor lizards, triggering trophic cascades that facilitated dramatic and sometimes unexpected increases in several prey of the predators, including smaller lizards, snakes, turtles, crocodiles, and birds. Persistence of isolated populations of these predators with a decades-long sympatry with toads suggests the possibility of recovery, but alternative explanations are possible. Confirming predator recovery requires longer-term study of populations with both baseline and immediate post-invasion densities. Previously, we quantified short-term impacts of invasive cane toads on animal communities over seven years at two sites in tropical Australia. Herein, we test the hypothesis that predators have begun to recover by repeating the study 12yr after the initial toad invasion. The three predatory lizards that experienced 71-97% declines in the short-term study showed no sign of recovery, and indeed a worse fate: two of the three species were no longer detectable in 630km of river surveys, suggesting local extirpation. Two mesopredators that had increased markedly in the short term due to these predator losses showed diverse responses in the medium term; a small lizard species increased by ~500%, while populations of a snake species showed little change. Our results indicate a system still in ecological turmoil, having not yet reached a "new equilibrium" more than a decade after the initial invasion; predator losses due to this toxic invasive species, and thus downstream effects, were not transient. Given that cane toads have proven too prolific to eradicate or control, we suggest that recovery of impacted predators must occur unassisted by evolutionary means: dispersal into extinction sites from surviving populations with alleles for toxin resistance or toad avoidance. Evolution and subsequent dispersal may be the only solution for a number of species or communities affected by invasive species for which control is either prohibitively expensive, or not possible.

Cane toads beneath bird rookeries: utilization of a natural disturbance by an invasive species.

Many invasive species exploit anthropogenically disturbed habitats, but most of those taxa evolved long before humans. Presumably, then, an ability to use natural (non-anthropogenic) disturbances pre-adapted invaders to a world later degraded by people. Studies on invasive species in naturally disturbed habitats thus can clarify the ancestral niche of invaders. In the Australian tropics, metallic starlings Aplonis metallica nest communally in emergent rainforest trees during the wet-season, and invasive cane toads Rhinella marina join other predators (mammals, birds, reptiles, and other anurans) to exploit the food resources beneath those trees. Compared to conspecifics found along nearby roads through the forest, cane toads beneath bird-nesting trees occur at higher densities, and are smaller in body size. The sex ratio is female-biased, and recapture records suggest that females may be philopatric at these sites (whereas recaptures were rare for both sexes found along the roads). Some toads were found under the same trees in successive wet-seasons. Spooling showed that distances moved per night were similar along the road versus under the trees, but toads under trees showed lower net displacements. Diets also differed (based upon scat analysis), with tree toads feeding more on beetles and less on ants. These nutrient-rich hotspots are exploited primarily by adult females and juvenile toads, whereas adult males congregate at breeding sites. By magnifying pre-existing intraspecific divergences in habitat use, bird rookeries may enhance population viability of cane toads by enabling critical age and sex classes to exploit food-rich patches that are rarely used by adult males.

New Weapons in the Toad Toolkit: A Review of Methods to Control and Mitigate the Biodiversity Impacts of Invasive Cane Toads (Rhinella Marina).

Our best hope of developing innovative methods to combat invasive species is likely to come from the study of high-profile invaders that have attracted intensive research not only into control, but also basic biology. Here we illustrate that point by reviewing current thinking about novel ways to control one of the world’s most well-studied invasions: that of the cane toad in Australia. Recently developed methods for population suppression include more effective traps based on the toad’s acoustic and pheromonal biology. New tools for containing spread include surveillance technologies (e.g., eDNA sampling and automated call detectors), as well as landscape-level barriers that exploit the toad’s vulnerability to desiccation—a strategy that could be significantly enhanced through the introduction of sedentary, range-core genotypes ahead of the invasion front. New methods to reduce the ecological impacts of toads include conditioned taste aversion in free-ranging predators, gene banking, and targeted gene flow. Lastly, recent advances in gene editing and gene drive technology hold the promise of modifying toad phenotypes in ways that may facilitate control or buffer impact. Synergies between these approaches hold great promise for novel and more effective means to combat the toad invasion and its consequent impacts on biodiversity.

Super-linear spreading in local and non-local cane toads equations

In this paper, we show super-linear propagation in a nonlocal reaction–diffusion–mutation equation modeling the invasion of cane toads in Australia that has attracted attention recently from the mathematical point of view. The population of toads is structured by a phenotypical trait that governs the spatial diffusion. In this paper, we are concerned with the case when the diffusivity can take unbounded values, and we prove that the population spreads as t3/2. We also get the sharp rate of spreading in a related local model.

The impact of transportation and translocation on dispersal behaviour in the invasive cane toad.

Biological invasions transport organisms to novel environments; but how does the translocation process influence movement patterns of the invader? Plausibly, the stress of encountering a novel environment, or of the transport process, might induce rapid dispersal from the release site-potentially enhancing (or reducing) invader success and spread. We investigated the effect of transportation and release to novel environments on dispersal-relevant traits of one of the world's most notorious invaders, the cane toad (Rhinella marina). We collected toads in northern New South Wales from heath and woodland habitats, manipulated the level of transport stress and either returned toads to their exact collection point (residents) or reciprocally translocated them to a novel site. Both translocation and the level of transport stress drastically altered toad dispersal rates for at least 5days post-release. Translocated toads (depending on their level of transport stress and release habitat) moved on average two to five times further per day (mean range 67-148m) than did residents (mean range 22-34m). Translocated toads also moved on more days, and moved further from their release point than did resident toads, but did not move in straighter lines. A higher level of transport stress (simulating long-distance translocation) had no significant effect on movements of resident toads but amplified the dispersal of translocated toads only when released into woodland habitat. These behavioural shifts induced by translocation and transportation may affect an invader's ability to colonise novel sites, and need to be incorporated into plans for invader control.

Super-linear spreading in local bistable cane toads equations

In this paper, we study the influence of an Allee effect on the spreading rate in a local reaction–diffusion–mutation equation modeling the invasion of cane toads in Australia. We are, in particular, concerned with the case when the diffusivity can take unbounded values. We show that the acceleration feature that arises in this model with a Fisher-KPP, or monostable, nonlinearity still occurs when this nonlinearity is instead bistable, despite the fact that this kills the small populations. This is in stark contrast to the work of Alfaro, Gui-Huan, and Mellet–Roquejoffre–Sire in related models, where the change to a bistable nonlinearity prevents acceleration.

Geographic divergence in dispersal-related behaviour in cane toads from range-front versus range-core populations in Australia

In invasive species, geographically variable evolutionary and ecological pressures can cause the rapid evolution of divergent behavioural phenotypes. Studies on invasive cane toads (Rhinella marina) in tropical Australia have revealed strong (and heritable) shifts in physiological traits related to dispersal rate. Behavioural phenotypes may have evolved in similar ways. We used standardised arena trials to test field-collected adult female toads from three populations: a range-core area in Queensland (ca. 76 years post-colonisation), a range-front population in Western Australia (<5 years post-colonisation) and an intermediate Northern Territory population (11 years post-colonisation). As predicted, toads from the range-front population were more exploratory and more likely to take risks in a novel arena environment than were conspecifics from the range-core population. We suggest that differential selection on behavioural responses to novel conditions in range-core versus range-front populations has produced a distinctive behavioural phenotype at the range-front that retains a high propensity for exploration and risk-taking (enhancing the ability of range-front toads to locate food and shelter) even when faced with novel environments. In contrast, at the range core where the locations of resources are known, a decrease in exploration and risk-taking in response to a novel environment may be favoured as it assists toads in evading threats. Ongoing biological invasions provide an ideal opportunity to examine which phenotypic traits drive establishment, range-expansion and invasion success. Furthermore, ongoing invasions allow us to investigate if variation in evolutionary and ecological pressures across an invasion range leads to geographical divergence in phenotypic traits. Dispersal ability is a key factor in invasion success. Behavioural traits such as exploration and a propensity to take risks enhance dispersal as individuals with these traits rapidly move out of their existing range and exploit new habitats and resources. We studied geographic divergence of dispersal-related behavioural traits across the Australian invasion range of cane toads (Rhinella marina) using standardised laboratory trials. We found that range-front toads were more exploratory and more likely to take risks than were conspecifics from range-core areas. Our results suggest that dispersal-enhancing behavioural traits may be important drivers of invasion success in cane toads.

Variation in Major Histocompatibility Complex diversity in invasive cane toad populations

Context The cane toad (Rhinella marina), a native species of central and southern America, was introduced to Australia in 1935 as a biocontrol agent after a complex history of prior introductions. The population rapidly expanded and has since spread through much of the Australian landmass, with severe impacts on the endemic wildlife, primarily via toxicity to predators. The invasion process has taken its toll on the cane toad, with changes in the immunological capacity across the Australian invasive population. Aims To investigate the immunogenetic underpinnings of these changes, we studied the diversity of the Major Histocompatiblity Complex (MHC) genes in introduced cane toad populations. Methods We studied the diversity of two MHC genes (the classical class I UA locus and a class II DAB locus) and compared these with neutral microsatellite markers in toads from the Australian site of introduction and the Australian invasion front. We also included toads from Hawai’i, the original source of the Australian toads, to infer founder effect. Key results Diversity across all markers was low across Australian and Hawai’ian samples, consistent with a reduction in genetic diversity through multiple founder effects during the course of the successive translocations. In Australia, allelic diversity at the microsatellite markers and the UA locus was reduced at the invasion front, whereas all three alleles at the DAB locus were maintained in the invasion-front toads. Conclusions Loss of allelic diversity observed at the microsatellite markers and the UA locus could be the result of drift and bottlenecking along the invasion process, however, the persistence of DAB diversity warrants further investigation to disentangle the evolutionary forces influencing this locus. Implications Through the use of different molecular markers, we provide a preliminary description of the adaptive genetic processes occurring in this invasive population. The extremely limited MHC diversity may represent low immunogenetic competence across the Australian population, which could be exploited for invasive species management.